Delignification and bleaching of cellulosic pulp containing protectorwith oxygen in the presence of alkali

ABSTRACT

A METHOD OF DELIGGNIFYING AND BLEACHING A CHEMICAL OR SEMI-CHEMICAL CELLULOSIC PULP COMPRISING THE STEPS OF INTIMATELY ADMIXING A FLOCCULATING TYPE OF PROTECTOR WHICH TENDS TO PRODUCE COHERENT FLOC IN THE PRESENCE OF AN HYDROXIDE OTHER THAN AMMONIUM HYDROXIDE, WITH PULP UNDER CONDITIONS WHICH ARE SUBSTANTIALLY FREE OF ANY HYDROXIDE OTHER THAN OPTIONALLY AMMONIUM HYDROXIDE. THE PROTECTOR COMPRISES UP TO 3% BY WEIGHT OF THE PULP ON A DRY BASIS. THE PROTECTOR-CONTAINING PULP IS SUBJECTED TO THE ACTION OF OXYGEN GAS IN THE PRESENCE OF AN ALKALINE MEDIUM AT ELEVATED TEMPERATURE SUPERATMOSPHERIC PRESSURE. THE ALKALINE MEDIUM IS CONSTITUTED BY A SUBSTANCE, PREFERABLY SODIUM HYDROXIDE, OTHER THAN THE FLOCCULATING PROTECTOR WHICH MAY COMPRISES AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP COMPRISING MAGNESIUM OXIDE AND A MAGNESIUM SALT.

United States Patent 3,740,310 DELIGNIFICATION AND BLEACHING OF CEL- LULOSIC PULP CONTAINING PROTECTOR WITH OXYGEN IN THE PRESENCE OF ALKALI Leon Smith, Petersfield, Springs, and Henry Hugo Myburgh, Valeriedene, Johannesburg, Republic of South Africa, assignors to South African Pulp and Paper Industries Limited, Johannesburg, Republic of South Africa, and lAir Liquide, Societe Anonyme pour lEtude et lExploitation des Procedes Georges Claude, Paris, France No Drawing. Filed Oct. 14, 1970, Ser. No. 80,739 Claims priority, application Republic of South Africa, Oct. 15, 1969, 69/7,269

Int. Cl. D21c 9/10 US. Cl. 162-65 6 Claims ABSTRACT OF THE DISCLOSURE A method of delignifying and bleaching a chemical or semi-chemical cellulosic pulp comprising the steps of mumately admixing a zflocculating type of protector which tends to produce coherent floc in the presence of an hydroxide other than ammonium hydroxide, with pulp under conditions which are substantially free of any hydroxide other than optionally ammonium hydroxide. The protector comprises up to 3% by weight of the pulp on a dry basis. The protector-containing pulpis subjected to the action of oxygen gas in the presence of an alkaline medium at elevated temperature and superatmospheric pressure. The alkaline medium is constituted by a substance, preferably sodium hydroxide, other than the flocculating protector which may comprise at least one substance selected from the group comprising magnesium oxide and a magnesium salt.

This invention relates to the delignification and bleaching of chemical and semi-chemical cellulosic pulps with oxygen gas. in the presence of an alkaline medium.

For the purposes of this specification, the term oxygen gas includes any gas containing free oxygen, such as air.

It is known that a pulp can be delignified and bleached by subjecting it in an alkaline medium to the action of oxygen gas. When the treatment is carried out at elevated temperature and pressure, good delignification and bleaching can be obtained but it has been found that the physical strength properties of the pulp are affected adversely.

In our French patent specification No. 1,387,853, we disclose the treatment of a chemical pulp with oxygen gas under pressure in an alkaline medium in the presence of a protector acting to preserve the physical and mechanical strength properties of the pulp. Our French specification No. 1,387,853 and our South African patent specification No. 68 3,771 disclose suitable protectors.

In our South African patent specification No. 6,629 68, we disclose the impregnation of a cellulosic pulp with a protector by immersion of the pulp in a mixture of NaOH and the protector, preferably MgCO prior to treatment with oxygen gas.

It has now been discovered that by using a dilferent procedure, the elfect obtained with a certain type of protector can be improved.

According to the invention a method of delignifying and bleaching a chemical or semi-chemical cellulosic pulp includes the steps of intimately admixing a flocculating protector with pulp under conditions which are substantially free of any hydroxide other than'optionally ammonium hydroxide, the protector comprising up to 3% by weight of the pulp on a dry basis; and subjecting the Patented June 19, 1973 protector-containing pulp to the action of oxygen gas in the presence of an alkaline medium at elevated temperature and super-atmospheric pressure, the alkaline medium being constituted by a substance other than the protector.

It has been found that a certain type of protector which is operative to inhibit degradation of the physical and mechanical strength properties of pulp during oxygen gas treatment, such as magnesium oxide and magnesium salts, such as magnesium carbonate, magnesium silicate, magnesium acetate, magnesium sulphate and magnesium chloride or a mixture of these compounds, tend to form coherent floc in the presence of an hydroxide other than amonium hydroxide. In this specification, a protector of this type is referred to as a flocculating protector.

It is a characteristic of such coherent floc, that when present in the proportions which are obtained in the oxygen gas treatment of pulp, it does not distribute regularly over the pulp fibres.

Although the specific fiocculating protectors referred to in the previous paragraph, havebeen found to be highly effective to inhibit degradation of the physical and mechanical strength properties. of pulp during oxygen gas treatment, it has been found that the best results possible with such a protector are not obtained if it is added to the pulp in the presence of an hydroxide other than ammonium hydroxide, since the resultant floc cannot readily be evenly and uniformly distributed over the pulp fibres. It has been discovered in accordance with the invention that if such a fiocculating protector is admixed With pulp under conditions which are substantially free of hydroxide other than optionally ammonium hydroxide, improved results after treatment with oxygen gas, can be obtained.

It has been found that ammonium hydroxide does not have an objectionable eifect tending to produce a floc with a fiocculating protector. In. the circumstances, ammonium hydroxide may be present during the admixture of the flocculating protector and the pulp. Reference to conditions or pulp as being substantially free of any hydroxide is not intended to exclude the presence of ammonium hydroxide and it is to be understood the ammonium hydroxide may or may not be present.

During admixture with the flocculating protector, the pulp may be at a low consistency not exceeding 5%.

After the intimate admixture of the low consistency pulp with the flocculating protector, the pulp may be thickened for the subsequent oxygen gas treatment.

Liquor extracted from the pulp during the thickening thereof subsequent to admixture of the substantially bydroxide-free pulp with the flocculating protector, may be recirculated and mixed with fresh pulp under conditions which are substantially free of any hydroxide, other than optionally ammonium hydroxide, prior to or simultaneously with the introduction of floeculating protector to such pulp. In this way, any loss of usable chemicals can be avoided or at least minimised.

The method according to the invention may be applied to an alkaline pulp which has been rendered substantially free of hydroxide and also to an acid pulp, such as a sulphite pulp, which has been preserved in a substantially hydroxide-free condition.

As applied to an alkaline pulp, the pulp which is admixed with the fiocculating protector may be obtained by pre-treating the alkaline pulp with an acidic medium to render the pulp substantially free from hydroxide; and washing the substantially hydroxide-free pulp under substantially hydroxide-free conditions. For admixture with the protector, the consistency of the Washed pulp may be adjusted to any suitable value depending on the solubility of the protector. Where the protector is substantially insoluble, such as magnesium carbonate, the consistency should not exceed 5%. With soluble protectors, or protectors which have been rendered partially soluble as described below, higher consistencies may be used.

The alkaline pulp may be acid pre-treated by treating it with a sufficient quantity of acid or by subjecting it to mildly acid bleaching or by treating it with liquor from an acid bleaching stage. Advantageously, the alkaline pulp may be acid pre-treated substantially in accordance with our South African patent No. 67/ 3,680, preferably with a 1% sulphuric acid or solution at a pulp consistency in the order of 5% at a pH in the range from 1.2 to 4.8 for a period of time in the range from 5 to 60 minutes. The acid pre-treatment may be conducted at ambient temperature, but it has been found that a temperature from 35 to 70 C. gives more rapid results. There is nothing inherent in the process that restricts the consistency of the pulp to a value of about 5% if suitable equipment is used.

The acid pre-treatment may be followed by treatment in any suitable washing system in which the acid pretreated pulp which is substantially free of hydroxide, is Washed with water and/ or With suitable recycled filtrate substantially free from free hydroxide, such as may be derived from a subsequent thickening stage. The consistency of the Washed pulp may be adjusted to any suitable low value into which a fiocculating protector, such as magnesium carbonate, is added to an amount of up to 3%, preferably from 0.25% to 3% by Weight of the pulp on a dry basis. The protector should be free from free hydroxide and may be added in dry form or may be suitably dispersed in an aqueous medium produced by a suitable purifying treatment or a treatment with complexing agents to render innocuous any interfering metals present in the protector. The aqueous medium may be derived from a subsequent thickening stage. Where an insoluble protector, such as magnesium carbonate, is used, the whole or a portion of the protector and/ or the low consistency pulp slurry may be pre-treated with a suitable medium, such as carbon dioxide or an ammoniacal liquor, to render the protector at least partially soluble. As stated previously ammonium hydroxide may be present Without any objectionable effect.

The pulp-protector mixture is admixed thoroughly under substantially hydroxide-free conditions in the mixing system until a uniform distribution of the protector in the pulp is obtained. The temperature at which admixture is carried is not normally critical and may be carried out at ambient temperature or at any other convenient or suitable temperature.

It has been found quite surprisingly, that once the pulp and the fiocculating protector have been intimately admixed under substantially hydroxide-free conditions and the protector has been thoroughly dispersed throughout the pulp mass and is substantially retained, the retention of the protector over the pulp fibres may be further enhanced by making the protector-containing pulp sut'ficiently alkaline, for example to a pH of 9 or higher. For this purpose, at least a portion of the fresh alkaline medium required for the oxygen gas treatment may be added to the protector-containing pulp. Alternatively, waste liquor from the oxygen bleaching stage may be recirculated for this purpose or other Waste alkali or ammoniacal liquors may be added to the protector-containing pulp.

After intimate admixture, the pulp may be thickened to a suitable consistency for the subsequent oxygen gas treatment. If necessary, the pulp may be thickened after any necessary dilution to a suitable lower consistency. Recycled filtrate may be used for such dilution. Surprisingly, very high retention of protector is possible.

The filtrate from this thickening operation may be used for dilution purposes in the pulp-protector mixing system and before thickening, and also for slurrying the protector. This filtrate may also be used for Washing purposes in the washing system as described above, in this way retaining as much of the protector in the pulp circuit as possible and minimizing the amount of water discharged from this mixing operation. 'It also advantageously allows a suitably low consistency to be used in the mixing operation.

The protector-containing pulp may be thickened to any suitable consistency, depending on the manner in which the oxygen gas treatment is to be carried out. If oxygen gas treatment is to be carried out in accordance with our South African patent No. 68/ 6629, the protector-containing pulp may be thickened to a consistency in the range from 16% to 67%.

Any equipment suitable for use in pulp-bleaching, such as for instance filters, diffusers and/or screw presses, may be used in the Washing and thickening operations described above.

The whole or the remainder of the required alkali, preferably to a total amount of 2 to 12% by weight of NaOH on a dry pulp basis, may then be admixed with the pulp prior to the oxygen gas treatment which may be carried out at an operating temperature of from C. to 135 C. and a pressure from 70 to 400 p.s.i. The alkaline medium is constituted by a substance other than the protector.

The pulp may be further bleached by any Well known method.

EXAMPLE The following numerical example is illustrative of the invention Without in any way limiting the scope thereof. In the example, concentrations are given as percentages by weight on a dry pulp basis, and oxygen gas pressures are those pertaining at maximum temperature during oxygen gas treatment. Pulp consistency is the ratio, expressed as percentage, of the weight of dry pulp to the weight of wet pulp. All tests have been carried out to TAP'PI speci'fications unless stated otherwise. Time for heating up to maximum operating temperature during oxygen gas treatment was 3 minutes, but shorter or longer times can be employed.

The properties of the following pulps are compared in the table below:

(a) Untreated, unbleached softwood (pine) kraft (i.e. alkaline) pulp of KAPPA number 25.1 (hereinafter referred to as Pulp A).

(b) The pulp of (a) above after acid pre-treatment in accordance with our South African patent No. 67/3680 under the conditions indicated below, washing, admixture with a mixture of 1% 'MgCO and and 6% NaOH essentially as described in the preamble of this specification with reference to our South African patent No. 6629/68 and subsequent treatment with oxygen gas under the conditions set out below. (Hereinafter referred to as Pulp B.)

ACID PRE-TREATMENT Temperature Ambient. Consistency 5%. Time (minutes) 60. Acid medium (percent S0 1%.

pH of pulp mixture 3.3.

OXY GEN GAS TREATMENT Temperature C Time (minutes) 30 Consistency "percent" 16.6 Pressure at max. temperature kg./cm. 12.0

(c) The pulp of (a) above after the same acid pro-treatment as in (b) above, Washing, admixture with 1% MgCO under substantially hydroxide-free conditions in accordance with the present invention, subsequently addition of 6% NaOH and the same oxygen gas treatment as in (b) above. (Hereinafter referred to as Pulp C.)

It is clear from the table above, that as far as brightness, colour reverision and freeness are concerned, the results obtained with Pulps B and C are very similar. However, the remarkable increase in the burst and tear factors and the number of double folds obtained with Pulp C in comparison with that obtained with Pulp B, illustrates the advantages to be obtained with the method according to the invention.

It may be remarked that Pulp C approaches the strength characteristics of the unbleached Pulp A but that the comparison between Pulps C and A is not so revealing as the comparison between Pulps C and B as discussed above.

We claim:

1. A method of delignifying and bleaching a chemical or semi-chemical pulp, comprising the steps of intimately admixing a fiocculating protector comprising a magnesium compound with pulp under conditions which are substantially free of any hydroxide other than optionally ammonium hydroxide, the protector comprising up to 3% by weight of the pulp on a dry basis; and subjecting the protector-containing pulp to the action of oxygen gas in the presence of an alkaline medium at elevated temperature and super-atmospheric pressure, the alkaline medium being constituted by a substance other than the protector. 1

2. A method as claimed in claim 1, wherein the flocculating protector comprises at least one substance selected from the group comprising magnesium oxide and a magnesium salt.

3. A method as claimed in claim 1, wherein the flocculating protector is admixed with the pulp at a consistency not exceeding 5%.

4. A method as claimed in claim 1, wherein the pulp is thickened for the oxygen gas treatment at a stage subsequent to the admixture with the flocculating protector.

5. A method as claimed in claim 1, wherein the pulp which is admixed with the flocculating protector is obtained by pre-treating an alkaline pulp with an acidic medium to render the pulp substantially free from hydroxide; and washing the substantially hydroxide-free pulp under substantially hydroxide-free conditions.

6. A method as claimed in claim 4, comprising the steps of recirculating liquor extracted from the pulp during the thickening thereof; and mixing said liquor from the pulp.

References Cited UNITED STATES PATENTS 3,384,533 5/1968 Robert et a1. 162-65 3,695,994 10/1972 Worster et al. 162-455 FOREIGN PATENTS 683,771 7/ 1969 South Africa.

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner 

